Technical Field
This invention relates to tip shroud assemblies of axial flow gas turbine engine compressors and, specifically, to such shrouds which recirculate air through the shrouds at the tips of airfoil in the compressor.
Background Information
Aircraft axial flow gas turbine engine compresses air in a compressor section, mixes the compressed air with fuel and combusts the resultant mixture in a combustor section, and expands the hot exhaust flow through a turbine section that, via one or more shafts, drives the compressor section. Overall engine efficiency is a function of, among other factors, the efficiency with which the compressor section compresses the air. The compressor section typically includes a low pressure compressor driven by a shaft connected to a low pressure turbine in the turbine section, and a high pressure compressor driven by a shaft connected to a high pressure turbine in the turbine section. The high and low compressors each include several stages of compressor blades and stators or vanes.
The high and low compressors each include several stages of compressor blades rotating about the longitudinal axis of the engine. Each blade has an airfoil that extends from a blade platform to a blade tip. The blade tips rotate in close proximity to an outer air seal referred to as a “tip shroud”. The tip shroud circumscribes the blade tips of a given stage. The blade platforms and the tip shroud define the radially inner and outer boundaries, respectively, of the airflow gaspath through the compressor. In order to maximize the efficiency of a gas turbine engine, it would be desirable, at a given fuel flow, to maximize the pressure rise (hereinafter referred to as “pressure ratio”) across each stage of the compressor.
As is well known by gas turbine engine practitioners, stall or surge is a phenomenon that is characteristic of all types of axial or centrifugal compressors that limits their pressure rise capability. During compressor operation, stall occurs when the streamwise momentum imparted to the air by the blades is insufficient to overcome the pressure rise across the compressor stage resulting in a reduction in airflow through a portion of the compressor stage. The flow leakage that occurs across the clearance gap between the compressor rotor blade tip and stationary casing endwall is one well known mechanism for reducing the total streamwise momentum through the blade passage, thus, reducing the blade pressure rise capability and moving the compressor closer towards the stall condition. Compressor stall is a condition in which the flow of air through a portion of a compressor stage ceases, because the energy imparted to the air by the blades of the compressor stage is insufficient to overcome the pressure ratio across the compressor stage. If no corrective action is taken, the compressor stall may propagate through the compressor stage, starving the combustor of sufficient air to maintain engine speed. Under some circumstances, the flow of air through the compressor may actually reverse direction, in what is known as a compressor surge. Compressor stalls and surges are very much unwanted.
Various forms of endwall treatments have been employed for enhancing compressors stall range, generally at the expense of compressor efficiency. Endwall treatments and designs utilizes the static pressure rise created at the compressor to recirculate high-pressure fluid to energize low momentum fluid along the casing, hereinafter referred to as endwall blockage. To energize the low momentum fluid, high-pressure fluid is channeled from the rear to the front of a compressor blade through a passage contained within the casing surrounding the compressor. The high-pressure fluid is then reinjected upstream of the blade to energize the low momentum fluid at the casing. Examples of such endwall treatments are disclosed and described in U.S. Pat. No. 5,607,284 issued Mar. 4, 1997 to Byrne et al., and U.S. Pat. No. 7,074,006 issued Jul. 11, 2006 to Hathaway et al.
The pressure gradient between high pressure downstream inlet ports and low pressure upstream passage outlet ports of the passages is not always sufficient to draw enough air into the passage. It is, thus, highly desirable to have an endwall treatment that is better able to operate sufficiently over a wide range of engine operating conditions to avoid stall and surge.